billing ZIP when buying w/ gift cards?

I know there are many similar threads with this, but I still wanted to clarfy something.
I checked the wiki and it said that I should be careful with the zip code if I'm using a gift card when filling out the billing address. can anyone explain that a little? I regestered it online so does it work now? And in case you were wondering I was gonna try getting from xp medical and babmino.

Since credit cards in the US don't require PIN numbers some places require a "billing" zip code to verify that you are the card holder. If the zip code you enter does not match the zip code that the bill is sent to the transaction is rejected. Some online sites have a check box for gift cards while other recognize that the card is a prepaid/gift card.